1. Field of the Invention
The present invention relates to a sewing needle structure for stitching a hidden nylon zipper, more particularly to a sewing needle structure for stitching a hidden nylon zipper of the size No. 5 or above.
2. Description of the Related Art
As implied in the name, a hidden zipper as shown in FIG. 1 is a zipper that interlocks a plurality of interdigitating teeth 3 of a cloth tape 2 by pulling a slider 1 upward, and the interdigiting teeth 3 are totally hidden by the cloth tape 2. Unlike the traditional zipper, the present invention does not expose the interdigitating teeth 3. The emphasis on the hidden zipper lies on the secure positioning of each tooth 3, and when the zipper is in closed position, the turnover edge 21 of the two cloth tapes must be tightly sealed and securely coupled. As the operation of the traditional hidden nylon zipper is performed by means of two needles and two threads. The current specification used for the hidden nylon zipper is mainly 75/11, and the fixing position for installing the slider is  2 mm from the top end, and the stopper is at  0.75 mm from the lower end. To arrange the two sewing needles in parallel after installation, the width between the two stoppers can be accommodated completely into the teeth body. The positioning section at the top end of the needle is ground to approximately 0.6 mm, and thereby its central distance is about 0.8 mm (0.4 mm+0.4 mm=0.8 mm) after the teeth are interlocked. The two sewing threads of the stitch coupling the teeth have the maximum width range of about 0.9 mm to 1.0 mm (including the diameter of the sewing thread). Undeniably, width in the range of about 0.9 mm to 1.0 mm to the teeth. For smaller hidden nylon zippers of the size No. 4 or No. 3 having a total tooth width of about 2.6 mm, and the width of the tooth body is about 1.15 mm, it can meet the actual requirement. However, if the user intends to use a No. 5 hidden nylon zipper (having a total tooth width of about 3.7 to 3.8 mm, the crown of about 0.6 mm, the diameter of monofilament tooth of about 0.68 mm, and the tooth body of about 2.52 mm) for the overcoat, purse, bedspread, etc., it is obvious that the operation of the two threads cannot meet the basic requirements for hidden zippers. The width of the two threads is about 0.9 to 1.0 and they are placed in the middle of the tooth body having the width of about 2.52 mm. Obviously, the curved teeth with the total tooth width of about 3.8 mm cannot be surely and securely coupled. Meanwhile, the turnover folded edge of the two cloth tapes in turn results in unseal opening situation. Of course, as shown in FIG. 2, we can reduce the grinding of the single edge of the two needles 10 at the top end of the positioning section 161 so that the central distance a between two needle posts 102 is increased slightly. As shown in FIG. 2, the width b between two sewing stitches 20 can be increased accordingly. However, we all know that the best way to tie the two threads of the stitch is to trisect the total tooth width. The total tooth width of about 3.7 to 3.8 mm is trisected into equal segments, and each segment is about 1.25 mm. Obviously, it is too large for the curved tooth 3 and not able to attain the secure and reliable interlocking effect because the width of each segment for the regular No. 3 hidden nylon zipper is only 0.86 mm. Particularly, the result of the trisection will make the stitches at the inner side of the sewing stitch 20 unable to lean against the crown 31 very closely (as shown in FIG. 3). It also makes the edge 21 of the turnover folded section of the two cloth tapes 2 unable to interlock and has the shortcoming of being in the opened form (as shown in FIG. 4). We can adjust the foregoing sewing stitches 20 at the inner side to a location closer to the edge of the crown 31, but the other sewing stitches 20 will shift as well. Therefore the two sewing stitches 20 will be skewed and not able to position the curved crown 3 with an even sewing force, and the teeth may have a phenomenon of being loosen and fallen apart more easily.
From the above, we learn that the conventional double-stitch double-thread sewing for the hidden nylon zipper cannot apply for the zippers of size No. 5 or above, and such shortcoming needs to be improved.
The primary objective of the present invention is to provide a sewing needle structure for stitching a hidden nylon zipper, wherein the sewing needle structure has at least three needles, which are parallel to each other. Those parallel needles are arranged with a gap having a minimum length within the maximum range evenly distributed along the interlocking teeth. The sewing needle structure having at least three needles fixes each seam between the interdigitating teeth, and the distance between the interlocking can be set to a minimum, such that the positioning of each tooth onto the tape by splicing could be more secure and reliable. Furthermore, the turnover folded edge of the two zipper tapes can be directly interlocked and definitely sealed, and thereby the nylon zipper attains the hidden effect with high artistic quality.
To make it easier for our examiner to understand the objective of the invention, its structure, innovative features, and its performance, we use a preferred embodiment together with the attached drawings for the detailed description of the invention.